Doppler type correlation system



May 28, 1968 G. H. STEVENS DOPPLER TYPE CORRELATION SYSTEM 5 Sheets-Sheet l Filed Oct. 24, 1966 TOZUE] May 28, 1968 G. H. STEVENS 3,386,095

DOPPLER TYPE CORRELATION SYSTEM Filed Oc. 2i., 1966 5 Sheets-Sheet 2 N fa f fb -faf Afro/2 Y May 28, 1968 G. H. STEVENS 3,386,095

` l DOPPLER TYPE' CORRELATION SYSTEM Filed Oct. 24, 1966 5 Sheets-Sheetl :5

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DOPPLER TYPE CORRELATION SYSTEM Filed om. 24, 1966 5 SlwffS-SheeL S fifa/i5 ABSTRACT F THE DESCLSURE A continuous wave Doppler radar system which is modulated by a pseudo-random code derives the Doppler characteristics of a particular target by correlation of the return signal with a delayed portion of the transmitted signal and which unambiguously determines range.

This invention relates to correlation systems and particularly to radar systems or the like that utilize the Doppler effect for providing information regarding moving objects.

The invention will be described as embodied in a radar system designed particularly for detecting and locating moving targets such as enemy soldiers or vehicles that are moving under cover of darkness or under cover of a forest, for example. In this embodiment the radar transmits a continuous wave carrier that is modulated by a pseudo-random code. By means of correlation and a combination of features described hereinafter, the radar operator hears a Doppler signal if the target is moving, and determines the range of the target without ambiguity. The character of the signal that is heard is Characteristic of the particular target and its speed, and the Doppler signal is so free from noise signal that it is readily heard without being confused or drowned out by noise. This substantial freedom from noise together with freedom from range ambiguity is an improvement over prior radal systems of this general type.

An object of the invention is to provide an improved Doppler type correlation system.

A `further object of the invention is to provide an improved correlation system that supplies a Doppler signal which is readily heard above any noise that may be present.

A still further object of the invention is to provide an improved Doppler type correlation system that provides the range of a target at all ranges without ambiguity and that supplies a Doppler signal representative of the target that is of substantially greater amplitude than any noise that may be present.

In the embodiment of the invention which Will be described the carrier Wave to be transmitted is a radio carrier wave that is phase modulated by a pseudo-random code which repeats at a comparatively high frequency. Each code changes in amplitude at a pseudo-random rate, the change being from +1 to -1 in the present example. When the code changes from +1 to -1 or from -1 to -l-l, the radar carrier is shifted 180 degrees in phase, i.e., the phase of the carrier is reversed. With this type of modulation, the code may contain a comparatively small number of bits (1023 bits in this example) so that it is practical to design a system in which the code repeats at a comparatively high frequency (at a frequency of approximately 6000 per second in this example). This makes it comparatively easy to lter out the noncorrelated signal from the Doppler signal heard by the radar operator. This is because no complicated filtering is required when the code repetition frequency is higher than the highest frequency in the Doppler signal for which the operator listens, preferably at least twice that of the highest Doppler frequency. In the present example, the highest Doppler frequency passed to the operators States Patent earphones is 1500 cycles per second. Also, in the present invention there is no range ambiguity because the code and the code only phase modulates the carrier whereby there is no STC action as explained hereinafter.

The invention will be described in detail with reference to the accompanying drawing in which:

FIG. -1 is a block diagram of a radar system embodying the invention;

FIG. 2 is a circuit diagram of the receiver mixer of FIG. 1 and the associated circuit for balancing out amplitude modulation from the phase modulated carrier;

FlGS. 3 and 4 are groups of graphs that are referred to in explaining the all range mode of operation of the system shown in FIG. 1;

FIG. 5 is a graph showing the correlation function for a certain code that phase modulates the carrier in the system of FIG. 1 Where no STC (sensitivity time control) is employed;

FIG. 6 is a corresponding graph showing the correlation function for a system employing STC;

FIG. 7 is a diagram of a suitable phase modulator that may be used in the system of FIG. 1.

1n the several figures like parts are indicated by similar reference characters.

Referring to FIG. 1, the radar carrier Wave is generated by an oscillator 10 which supplies the carrier to a phase modulator 11. The carrier Wave is designated cos wot.

The oscillator 10 may supply a carrier having a frequency in the X band, or having Ia lower frequency if preferred. lf in the X band, the unit 10 may be a stable oscillator that feeds into a non-linear device to generate harmonics; a high frequency harmonic being sharply iiltered to pass a very clean X band carrier signal (a 9250 megacycle per second signal, for example) to the phase modulator 11.

At the phase modulator 11 the carrier is 180 degree phase modulated by a pseudo-random code supplied from a driver 12. This code is designated f(t). A specific example of a suitable phase modulator will be described later. The phase modulated carrier is supplied over a transmission line 13 to the first port of a three-port circulator 14, and passes out of a second port over a transmission line 16 to an antenna 17 which preferably is directional. The phase modulated signal from the modulator 11, which is the signal transmitted, may be Written f(t) cos wat. Some of the signal from the phase modulator leaks through the circulator to the third port and feeds through a transmission line 18 and feeds with reected signal, as discussed below, into a receiver mixer 19.

Reflected signal from `a target is picked up by the antenna 17. The round trip time of the radio wave transmitted to a target and reilected back to the radar set is designated f. Therefore, the reflected signal picked up by the antenna is pfU-i-f) cos w00t-H) where p is an attenuation constant. The reflected signal feeds over the line 16 to the circulator 14 Where it adds to the leakage referencey signal. This sum signal feeds from the third port through the line 18 into the mixer 19 Where the sum signal f() cos w0t+pf(t+r) cos woU-l-r) is squared.

The transmission lines 13, 16 and 18, as well as the lines interconnecting other units of the system, may be coaxial lines, for example.

Returning now to the pseudo-random code that is fed to phase modulator 11, it is generated by a pseudo-random code generator 21 known in the art. The code -is illustrated schematically -by the graph 9. It changes in amplitude from -i-l to -1. lf preferred, the code amplitude may be made to change in amplitude from -i-l to 0, for example. The code generator illustrated comprises a 10Jbit or lll-stage shift register 22 that feeds signal from stage 8 and from stage l to a multiplier 23. The multiplier output feeds back to stage 1 of the shift register. The shift register is driven by a clock 24 or master oscillator operating at 6 megacycles per second. Note that l/G me. is the transit time to a target at 25 meters and return. In this example, the code is 1023 bits long before it repeats, there being one bit for each pulse from the clock. Therefore, the code repeats at a comparatively high frequency which is equal to the clock frequency divided by the number of bits in the code. Thus the pseudo-random code repetition is 6,000,000/102326000` per second approximately. The pseudo-random code is taken off the first stage of the shift register and supplied over lines 26 and 27, and through switch Contact 2S and switch arm 29 to the driver 12 when the switch arm 29 is in what is referred to as the range bins position. The driver l2 may be an amplifier or any suitable circuit for increasing the amplitude of the code sufficiently for operating the phase modulator 11.

The code is also taken off the shift register yby way of one of the ten taps marked to and a tap switch 3l. The taps 1 to 10 take signal off shift register stages ll to 10, respectively. At tap 1 the code is delayed with respect to the undelayed code on line 26 by an amount equal to the transit time of the rada-r signal traveling to a target meters from the radar and returning to the radar. The code appearing on tap switch Slt is delayed an additional amount each time it -is moved to a higher number tap, the delay from one tap to the succeeding tap being equal to the radar transit time to and return for a target 25 meters from the radar. Thus, at tap 10, for example, the code delay is equal to the transit time of the radar signal going to and returning from a target 250 meters from the radar set. The amount the code is delayed is designated T.

The delayed code f(+T) from the tap switch 3l is supplied over a line 32 to a balanced mixer or multiplier 33. The undelayed code f(t) is supplied over a line 34 to the multiplier 33 also. The unit 33 may be any suitable device or circuit that multiplies the two input signals to give the product signal f(t)f(t+T). This product signal appearin-g at the output of mixer 33 is supplied by way of a contact point 36 and a switch arm 37 over a line 3S to a correlator 38 when the switch arm is in the range bins position.

The output of the receiver mixer i9 is supplied through a video amplifier 39 and a line 4l to the correlator 3S. As will be explained later, the signal fed to the correlator by line 41 is the product of the undelayed code f(t), the transit time delayed code f(t+), and a component cos wor representing Doppler signal due to target movement. That is, the signal on line 41 is f(t)f(t+'r) cos wof. This is the same signal as the reference signal f(t)f(t+T) supplied to the correlator over the line when T equals r, except for the Doppler component.

The output of the correlator 3S is supplied to an audio amplifier el which passes audio signal up to 1500 cycles per second in the present example. That is the Doppler signal cos vor which appears as the correlator output when the tap switch 31 is set on a shift register tap that supplies code that is delayed by substantially the same amount as the received radar signal, that is, when the code delay T equals or substantially equals the radar signal delay T.

The output of audio amplifier 42 is supplied to a utilization device such as ear phones indicated at 43. In operating the radar, the operator listens to the ear phones, points the antenna in the direction of a suspected moving target, and moves the tap switch 31 to the particular tap that results in the maximum amplitude signal in the ear phones, assuming the radar signal strikes a moving target so that a Doppler signal is obtained. This particular' tap gives the range of the target, the taps l to 10 representing ranges of 25 meters, 50 meters, etc., up to 250 meters, respectively. The direction -of the target is indicated by the direction in which the directional antenna is pointing.

The reason that the signal f(t)f(t+r) cos w01- appears on the lead 4l. will now be explained. First, reference is made to FIG. 2 which shows a suitable balanced mixer circuit 19. The diode 19a is the one of present interest, the diode 19h being for balancing out any amplitude modulation that may be present as will be described later. The diode 19a is provided with a load resistor 51, the upper end of which is connected to a summing resistor 52 (summing for ampiltude modulation cancellation). A tap 53 supplies the output of diode 19a (less amplitude modulation) through a coupling capacitor 54 to the video amplicr 39.

The sum signal or current e from the circulator is f() cos w0t+pf(f+) cos wDU-l-T). This is squared by the diode 19a. The current output of diode E9n is Since the trigonometric identiy 1/2(l+cos Zwot) may be substituted for cos2 met and a substitution also may be made using the trigonometric identity cos A cos B=V2 cos (A+B)+1/2 'cos (A-T), and since U) is either +1 or l whereby I2C) is always +1 and f2(t+r) is always +1, e2 may be written as follows:

The corresponding voltage V appearing at the tap 53 is a constant C times e2. The voltage appearing at the output of the video amplifier 39 is only the amplified component Cpf(l')f(t+r) cos wO--r because the other components of the squared signal are either direct current components or components having a frequency twice that of the X band carrier frequency, none of which is passed by the video ampliiier. Thus, the signal supplied over the line t-l to the correlator is f(t)f(t+T) cos w07 (times a constunt) as previously stated.

This video `amplifier ouput signal on line 41 should be regarded as containing Doppler because T, the round trip time, is not a xed constant number if the target is moving; in fact is a function of time, that is, the round trip time from the antenna to the target and back will differ from time to time. Therefore, we -may write 7(1) which is contained in the phase of the cosine term as a power series function, and expand it to write TU) as a constant term designated To (a xed quantity) plus a linear term which may be written Tot. Thus we may express T(t) as a constant time plus a linear function of time plus higher order terms as follows: T(t)=T9+i"Gt+1/zi0t2+ Ne are only concerned here with the Doppler itself which is the linear portion v0.1'.

The reference signai applied over line 35 to the correlator is f(f)f(t+T). Since Aa correlator multiplies the two :applied signals, its output is f2(t)f(t+v)f(t+T) cos wor. When the tap switch Iii is set so that T=1, this output is f2(t)f2(t+1) cos war. Since fitr) and f2(t+1) are each equal to +1, this contracts to cos w37- which is the Doppler frequency signal that is representative of a moving target. When T is not equal to T, the filtering or integration which is included in the correlator eliminates the broad spectrum signal.

A target may be located more quickly by first having the switch arm 29 set on a contact point 46, and the switch arm 37 set on a contact point 47. This is the all range position of these switch alims for the all range mode of operation. The contact point 46 is supplied from a frequency divider 48 with a square wave voltage recurring at 150,000 cycles per second. The divider 48 is driven by the 6 `megacycle clock 2li.

The Contact point 47 is supplied from a frequency divider 45? with a square wave voltage recurring -at 300,000 cycles per second. The divider 49 also is driven by the clock 24.

By using the all range mode, the operator may quick- 1y determine whether there is a moving target in the direction in which the antenna is pointing and anywhere Within the maximum radar range, within 250 meters in lthis example. The operation of this mode will be explained with reference to the graphs in FIGS. 3 and 4.

lReferring to FIG. 3, graph (a), this is the 150,000 cycles per second square wave which is the code f(t) that phase modulates the carrier wave in the all range mode operation. This is :a 180 degree phase shift modulation just as in the range bins mode. This repetition rate or frequency of the square wave is selected so that the time of one-fourth cycle of the wave is equal to the transit time out and back for a target at the maximum range, at 250 meters in this example. Thus, the received reflected signal from such a target is f(t'l-T) and is delayed as indicated by FIG. 3, graph (b). As previously described, and as indicated in FIG. l, the output of the video amplifier 39 is f(t)f(t|1) times the Doppler signal cos wor. To simplify the graphs in FIGS. 3 and 4, it is assumed that the targets are stationary so that there is no Doppler signal. FIG. 3, graph (c) is the product of graphs (a) and (b), and thus represents the video amplifier output which is fed into the correlator 38. This same signal should be fed into the correlator over line 35 to obtain complete correlation for signal from a target at maximum range.

Such a signal, which is fed over line 35, is shown in FIG. 3, graph (f). It is the 300,000 cycle per second square wave supplied from frequency `divider 49. Referring to FIG. 3, graphs (d) and (e), it will be seen that it corresponds to the product of the code f(t) land the delayed code f(l-{- T) Where T :7, lr in this case being the transit time Where the target is at maximum range.

This all range7 mode provides a sensitivity time control (STC) elfect because the correlation becomes less and less complete as the target gets closer to the radar set. Maximum Doppler signal :amplitude is heard when there is complete correlation.

This difference between complete and partial correlation is illustrated in a general way by the graphs of FIG. 4. Refer first to FIG. 4, graphs (f), (g) and (h), which illustrate the complete correlation which occurs when the target is at the 250 meter maximum range. Graph (f) represents the signal fed over line 41 into the correlator from the video amplifier. Graph (g) represents the 300,- 000 cycle signal fed over line 3S to the correlator, where the delay T equals the delay f. The product of these two signals (after multiplication by the correlator) is s-hown by graph (h). Since a stationary target has been assumed, this correlator ouput is direct current of a certain value indicated as -l-l. The presence of Doppler signal would appear as a variation in the :amplitude of this direct current.

Refer next to FIG. 4, graphs (a), (b), (c), (d) Vand (e), which illustrate the partial correlation that occurs for a target at 125 meters from the radar set. Graph (c) is the product of the code f(t) shown in graph (a) and the received reiiected code f(t-}1) shown in graph (b). This product signal is fed over the line "el to the correlator.

The other signal fed to the correlator, by way of line 35, is the 300,000 cycle wave shown by graph (d). The two signals, graphs (c) and (d), fed to the correlator are multiplied in the correlator to give the product signal shown by graph (e). After the filtering or integration occurring in the correlator, the correlator output is a direct current of reduced value as compared with the value when there is the complete correlation illustrated in graph (h). It will be evident that any direct current variation representing Doppler signal cannot be as great in the case of partial correlation, graph (e), as in the case of complete correlation, graph (h), providing the larger direct current amplitude.

In the use of this all range mode, the operator will hear a Doppler signal if there is a moving target anywhere Within the 250 meter range. Therefore, in this mode the operator swings the antenna until he hears a Doppler signal. He then knows the antenna is pointing to a moving target. He then switches to the range bins mode, and moves the tap switch 31 over the taps 1 to 10 until maximum amplitude Doppler signal is heard. The tap on which switch 31 is located is then noted and the corresponding range is read olf.

The above-described correlation system whether operating in the range bins mode or in the all range mode requires no elaborate filtering, such as the use of comb filters, and supplies a substantially noise free Doppler signal to the ear phones. The reason for this is that when the repetitive code directly phase modulates the carrier wave to shift its phase degrees as described, the frequency spectrum of the resulting signal consists of frequency components: zero plus target Doppler frequency, code repetition rate plus and minus Doppler frequency, two times code repetition rate plus and minus Doppler frequency, etc. Therefore, this resulting signal consists of frequency components spaced apart by a frequency equal to the code repetition rate minus twice the target Doppler frequency (which in this example is 6000 per second). Thus, in the present example the demodulated carrier applied to the video amplifier has a frequency spectrum having frequency components at the target Doppler frequency, at 6000 cycles per second plus and minus the Doppier frequency, at 12,000 cycles per second plus and minus the Doppler frequency, etc. In the present example, the highest frequency Doppler signal to be passed to the ear phones is one of 1500 cycles per second. Therefore, by designinfr the audio amplifier (or by inserting a separate filter) to pass signal from about 30 cycles per second to slightly above i500 cycles per second, as in this specific example, and to exclude signals in the region of 4500 cycles (6000-l500 cycles) and above, the noise components due to uncorrelated signals, and particularly those due to the code repetition rate, are eliminated.

From the foregoing it will be apparent that not only should the code repetition frequency be higher than the highest Doppler frequency to be utilized, but preferably it should be at least two times said highest Doppler frequency to avoid overlap in the frequency spectrum of the Doppler frequency and the code repetition frequency (6000 in this example) minus the doppler frequency.

One of the important features in the combination of features in the above-described correlation system is the fact that the modulating signal applied to the phasemodulator is the code and only the code, that is, the modulating signal applied to the phase modulator is not the product of the code and another signal as for example, an intermediate frequency (IF) carrier signal modulated by the code. A code modulated by an IF carrier is the modulating signal that is used in Doppler correlation systems employing sensitivity time control (STC). In the present invention no STC is employed. In the present invention this is important as will be described in connection with FIGS. 5 and 6.

FIG. 5 shows the correlation function for the code in the present invention where STC is not employed. This function is illustrated for the particular example that has been described where the code is 1023 bits long, and where the duration of a code bit is 1/ 6 Inc. (note that the clock frequency is 6 mc.).

FIG. 6 shows the correlation function for the same code as in FIG. 5, but for the case where the correlation system employs STC. When STC is employed the code is multiplied or modulated by another signal (such as an IF carrier) and this product modulates the phase modulator. This multiplication or modulation destroys the ideal correlation function of the code as is evident from a come parison of FIGS. 5 and 6. In the present invention the code has the ideal correlation function shown in FIG. 5. An example of a Doppler type correlation system ernploying STC is shown in FIG. l1 of an article entitled '7 Correlation Radar Using Pseudo-Random Modulation by W. Fishbein and O. E. Rittenbach appearing in the 1961 International Convention Record, pt. 5, pp. 259-277.

In FIGS. and 6 the abscissa is T T which are the transit time and the code delay time, respectively, as defined in connection with FIG. 1. At zero (where TIT) there is complete correlation, and a Doppler signal of maximum amplitude is heard, this amplitude being represented as l in FIG. 5 and as R/Rmax, in FIG. 6, where Rzrange of target and where Rmax'zmaximum range of target. In the case of FIG. 6 where STC is employed this maximum signal amplitude is decreased by the STC action as the target range R decreases. Incidentally, the quantity R/Rmxmay be to the first power, or to some other power such as 2, depending upon the STC design.

In the correlation function of FIG. 6 the time sidelobes do not change in amplitude with target range. They have the amplitudes A1, A2, A3, etc., which, in the present example, are between a minimum of $4.023 and about 79/1023, being independent of target range. if the amplitude R/Rmx. becomes equal to or less than any one of the time sidelobes it is not possible to distinguish between the desired Doppler signal and a time sidelobe, and the target range is lost because of range ambiguity. There is no such range loss in the present invention where the code has the ideal correlation function as shown in FIG. 5.

In the present invention where the code and only the code produces 180 degree or phase-reversal modulation of the carrier Wave (there being no STC), and where the code repetition frequency is higher than that of the highest frequency Doppler signal to be utilized, i.e. to be passed to the ear phones, there is no substantial amount of noise present within the frequency band of the Doppler signal utilized and the range of the target can be determined without ambiguity. All severe noise is outside the frequency band of the Doppler signal and is easily filtered out. It will be recognized that some clutter noise may be present, but this is minor, and is minor at all target ranges. The desired Doppler signal is always readily heard above any kind of noise that may be present.

Refer now to the balanced circuit of FIG. 2 which cancels out any amplitude modulation if such modulation is introduced by the phase modulator l1. A small portion of the transmitted signal is taken off the line 16 by a coupler 56 and fed to'a radio frequency attenuator 57 which is adjusted to supply signal of the proper strength to the diode 1917. The diode 19b is connected in polarity opposition to the mixer diode 19a so that the signal appearing across itsload resistor 53 is of opposite polarity to that appearing across the load resistor 51 of diode 19a. The resistors 51 and 58 may have a resistance of 220 ohms each, for example.

The signal appearing across load resistor 58 is applied to the upper end of the sum resistor 52. Thus it tends to cancel the corresponding opposite polarity signal applied to the lower end of sum resistor 52. The resistor 52 may have .a resistance of 500 ohms, for example.

Referring more specifically to the cancellation operation, and taking certain values by way of example, assume the input power to the circulator by way of conductor 13 is 6 dbm. (Note that Zero dbm.=1 rnilliwatt.) The circular leakage attenuation is about -18 dbm., so that the detected amplitude modulation at the load resistor 51 is l2 dbm.

The transmitted signal picked up by the coupler 56 is attenuated about -14 dbm. by the coupler and the RF attenuator 57 so that the detected amplitude modulation at the load resistor 58 is -8 dbm. Thus, the voltage of the amplitude modulation is greater at the top of sum resistor S2 than at the bottom resulting in an amplitude modulation balancing out position for tap 53 toward the bottom of the resistor 52. This is desired so that the Doppler signal output of mixer diode 19a will pass through a minimum amount of the sum resistor and thus suffer minimum attenuation.

The Doppler signal output of diode 19a is not balanced out by the Doppler signal output of diode 19h because the reflected signal that reaches diode 19h is greatly attenuated (-14 dbm. in this example), whereas the reflected signal reaching diode 19a through the circulator is not attenuated.

It may be noted that the input end of the video amplifier provides a load impedance (of about 1000 ohms in this example) into which the signal from tap 53 feeds. A capacitor 61 connected between the tap 53 and ground functions as an envelope detection filter cooperating with the amplitude detection function of the diodes 19a and 1911.

It will be evident that any amplitude modulation that may be present may be balanced out by adjusting the tap 53 to the proper position.

The specific example of a suitable phase modulator (modulator 11) previously referred to will now be described.

Refer to FIG. 7.

The phase modulator comprises a three port circulator 75, a short transmission line 76 leading from the second port, and a varactor diode 77 that, together with an inductive impedance described later, terminates the transmission line 76. The 9250 me. input signal from the oscillator 1G (FIG. l) is applied to the first port through a transmission line 74- in this case a -coaxial line.

It is a characteristic of a circulator that improper termination of a port causes reiiection of part of the signal into the next port (in the direction of the arrow on the circulator 75). If the termination is reactive, which is the case here where the termination is either inductive or capacitive depending upon the varactor capacity, the energy is completely reflected from the second port to the third port and out the line 13. Also, the change in phase of the reflected signal is a function of the change in the capacity of the terminating varactor 77.

The varactor diode 77 is back-biased sufficiently by a bias voltage source (not shown) so that when the code signal is applied the varactor is back-biased during both the -i-l value and the -1 value of the code and, therefore, has a certain capacitive value for each code value. The varactor has one capacity at the -l-l code value resulting in a phase shift of one value for the reflected carrier, and it has a different capacity at the -l code value resulting in a phase shift of a different value for the reflected carrier. These two phase shift values are made to differ by degrees.

In the example being described, the transmission lines 74, 13 and 76 are coaxial lines having a characteristic impedance of fifty ohms. The circulator 75 also has a characteristic impedance of fifty ohms.

The transmission line 76 may be considered to be terminated at the junction point 78 where varactor 77 is connected to the `center conductor 79. The length of line 76 (from the circulator to the junction point 78) is not critical. In the present example it is between 1A; and Mt wavelength long, but it may be 1/zx or more in length. The wavelength )t is the wavelength of the 9250 mc. carrier.

A coaxial line section 79 extends from the line 76 for the purpose of adding inductance to the termination of line 76. The end of line 76 is short circuited. The point of short circuiting is made adjustable by an adjustable direct-current short circuit connection 81. The coaxial line length from the junction point 78 to the short circuit S1 is less than MM so that the impedance looking into the line section 79 from junction point 78 is inductive.

As described, one side of varactor 77 is connected to the center conductor 79. The other side of the varactor is connected to the center conductor 82 of a coaxial line 83 through which the code signal is fed to the varactor. The code is applied through a direct-current amplifier (not shown), the proper back bias being applied to the varactor. it will be noted that the code is applied to the varactor through the center conductor 82; and through the outer conductor of line 83, the outer conductor of line 79, and through the short 81 and the center conductor of line 79 to the other side of the varactor.

An adjustable alternating-current shorting connection 84 is provided in the line 83. This short is made A-C instead of D-C to avoid shorting out the code signal. The short 84 is positioned so that the line 83 looking into it from the varactor is capacitive. The short 84 is adjusted to a point where it effectively removes the inductance and stray capacity associated with the varactor, i.e., the line capacity provided by the short and the reactance associated with the varactor are made to resonate at 9250 From the foregoing it will be seen that the line 76 is terminated by both capacity and inductance, the capacity being provided by the varactor 77, and the inductance being provided by the shorted line section 79.

The above-described phase modulator may be adjusted to provide the desired 180 degree phase shift as follows. The code signal with the back bias is adjusted so that at one code value (the +1 code value, for example) the back bias voltage has a selected maximum value so that the varactor has a selected minimum capacitance value, and so that at the other code value (-1 in this example) the varactor has a selected maximum capacitance value. This change in varactor capacity may be in the order of a two to one or a three to one change.

The code signal values corresponding to |1 and -1 are now supplied to the line S3 by manual switching. First the +1 value signal is applied so that the varactor has the selected minimum capacity value. Under this condition, looking into the line 76 from the circulator the line 76 looks predominately inductive, and the line 76 appears to have a length L0.

Next the -l value signal is applied so that the varactor has the selected maximum capacity value. Now the line 76 looks predominately capacitive looking into it from the circulator, and the line 76 appears to have a length L1, the length L1 being greater than the length L0. The difference between the lengths L and L1 is adjusted until this length difference is exactly Mm, thus, providing a 180 degree phase shift since the carrier is reected, i.e., it passes down the line 76 and back to the second port. Therefore, in this round trip the carrier passes lthrough a length of line that is l/zx greater when the line length is L1 than when it is L0.

This desired difference in the line lengths L1 and L0 is obtained by adjusting the position of the shorting bar S1. The correct position of the short 81 is determined by means of a phase meter connected to the input line 74 and the output line 13. When the phase difference between the input signal and the output signal is shown to be 180 degrees, the adjustment is complete.

In the course of adjusting the D-C short S1 to the correct position, the A-C short 84 usually should also be adjusted. It is known that the A-C short 84 is at its proper position when the D-C short 81 becomes effective so that its adjustment results in the desired 180 degree phase shift.

Phase modulation by the above-described phase modulator produces little or no amplitude modulation. This is because the 180 degree phase shift is obtained by a varactor capacitor change (such as two or three to one) that is small enough so that the varactor is substantially entirely capacitive in operation, any lossy resistive effects being negligible. It may be noted also that the coaxial lines 76, 79 and 83 have a low internal loss.

What is claimed is:

1. A correlation system which comprises means for generating a repetitive code which varies in amplitude and which has a certain repetition rate, means for generating a carrier wave,

means including a phase modulator to which a modulating signal consisting of said code and only said code is applied for reversing the phase of said carrier wave with each change in code amplitude,

means for transmitting said phase modulated carrier toward a target,

means for receiving said carrier wave after reflection from said target, said received reflected signal being delayed by the transit time,

a receiver mixer,

means for supplying to said receiver mixer said reected signal and a reference signal consisting of an attenuated portion of said phase modulated carrier prior to transmission,

a correlator,

means for supplying to said correlator the component of said receiver mixer output containing Doppler signal,

means for applying to said correlator a comparison signal that corresponds to the product of said code delayed a selected amount and said code undelayed whereby a Doppler signal representative of said moving target appears in the output of said correlator when said selected code delay is substantially equal to the transit time of the received reliected signal,

a utilization device, and

means for supplying to said device only that portion of said correlator output which has a frequency below -a certain frequency limit,

said certain repetition rate of said code being higher than the frequency of said frequency limit.

2. A correlation system which comprises means for generating a repetitive code which varies in amplitude and which has a certain repetition rate,

means for generating a carrier wave,

means including a phase modulator to which a modulating signal consisting of said code and only said code is applied for reversing the phase of said carrier wave with each change in code amplitude,

means for transmitting said phase modulated carrier toward a target,

means for receiving said carrier Wave after reflection from said target, said received reflected signal being delayed by the transit time,

a receiver mixer,

means for supplying to said receiver mixer said reflected signal and a reference signal consisting of an attenuated portion of said phase modulated carrier prior to transmission,

a correlator,

means for supplying to said correlator the component of said receiver mixer output containing Doppler signal,

means for delaying said code a selected amount to produce a delayed code,

means for multiplying said delayed code by said code undelayed to obtain a resulting product signal,

means for applying said product signal to said correlator whereby a Doppler signal representative of said moving target appears in the output of said correlator when said selected code delay is substantially equal to the transit time of the received reflected signal,

a utilization device, and means for supplying to said device only that portion of said correlator output which has a frequency below a certain frequency limit,

said certain repetition rate of said code being at least twice the frequency of said frequency limit.

3. A correlation system lwhich comprises means for generating a repetitive code which varies in amplitude and which has a certain repetition rate,

means for generating a carrier wave,

means including a phase modulator to which a modu lating signal consisting of said code and only said code is applied for reversing the phase of said carrier wave with each change in code amplitude,

an antenna,

a circulator,

means for transmitting said phase modulated carrier through said circulator to said antenna and toward a target,

a receiver mixer,

some of said transmitted modulated carrier being sup` plied to said mixer by way of leakage through said circulator,

means for supplying said modulated carrier after reection from a target and after being picked up by said antenna through said circulator to said receiver mixer, said received reflected carrier being delayed by the transit time,

a correlator,

means for supplying to said correlator the component of said receiver mixer output containing Doppler signal,

means for applying to said correlator a comparison signal that corresponds to the product of said code delayed a selected amount and said code undelayed whereby a Doppler signal representative of said moving target appears in the output of said correlator when said selected code delay is substantially equal to the transit time of the received retlected signal,

a utilization device, and

means for supplying to said device only that portion of said correlator output which has a frequency below a certain frequency limit,

said certain repetition rate of said code being at least twice the frequency of said frequency limit.

4. A correlation system which comprises code generating means for generating a repetitive pseudo-random code which varies in amplitude and which has a certain repetition rate,

said code generating means including a shift register comprising a plurality of stages,

said shift register having output taps from successive stages,

means for generating a carrier wave,

means including a phase modulator to which a modulating signal consisting of said code and only said code is applied for reversing the phase of said carrier wave with each change in code amplitude,

means for transmitting said phase modulated carrier toward a target,

means for receiving said carrier wave after reection from said target, said received reflected signal being delayed by the transit time,

a receiver mixer,

means for supplying to said receiver mixer said reflected signal and a reference signal consisting of an attenuated portion of said phase modulated carrier prior to transmission,

a correlator,

means for supplying to said correlator the component of said receiver mixer output containing Doppler signal,

means for taking said code off a selected one of said shift register taps for obtaining a code delayed a selected amount,

means for multiplying said delayed code by said code undelayed to obtain a resulting product signal,

means for applying said product signal to said correlator whereby a Doppler signal representative of said moving target appears in the output of said correlator when said selected code delay is substantially equal to the transit time of the received reflected signal,

a ultilization device,

and means for supplying to said device only that portion of said correlator output which has a frequency below a certain frequency limit,

said certain repetition rate of said code being at least twice the frequency of said frequency limit.

No references cited.

RODNEY D. BENNETT, Primary Examiner'.

40 l. P. MORRS, Assistant Examiner. 

